Formula for Horsepower

[Mule]

So if an engine makes say 320, like an S4, it does this on atmospheric pressure of 14.7 pounds per sq. in. Power is limited by how much air can flow through the engine. 1 bar or 14.7 pounds per sq. in. of boost basically doubles the amount of air and the amount of gas that can be burned and the amount of cylinder pressure that can be generated.

There are some losses in friction and efficiency and cost to turn the compressor (nearly zero with a turbo). So if the engine makes 320 at a high level of efficiency and you add 1 bar while maintaining that level of efficiency, you make 640. I believe the laws of physics govern this reliably. This is how they are able to rate fuel pumps as "capable of supporting X hp." I don't believe there is any way around this.

Can anyone explain how they produce numbers exceeding this ratio?

[Danglerb]

Marketing.

I am a big believer in pounds of air through the motor equals power out minus a bit of friction.

There is also plenty of confusion from freely going back and forth between flywheel HP, ie S4 and 320, and rear wheel HP, ie S4 about 260 rwhp.

Also I am a big believer that nothing has increased HP more than the use of Dynojet's to measure it.

[DanielDudley]

My S4 put put out 286 RWHP on the dyno. No X pipe, cats and stock muffler. I believe it would have topped 300 with those mods and a RRFPR. Mustang dyno.

[rhjames]

what pump are you specifically trying to debunk?

fuel pump, air pump, supercharger, turbocharger, etc.????

was your question was more of an engineering/physics question rather than dealing with a specific application or product?

the example of doubling the amount of air/fuel with 1 bar (14 psi) is not exactly true, as the fuel supply is not doubled just by adding a turbo or supercharger, unless the fuel supply is BEFORE the sc or tc (in other words, on the intake side of the huffer).

you will see, however, that 928 Specialist's sc doesn't quite make twice the HP, nor does Carl Faussets' belt drive sc, nor does Mark Robinson's turbo kit. then again, maybe I'm so tired I've read it wrong......

the one thing that none of these kits do (according to what I've read and seen----disclaimer/ of course, I've been mistaken before), is address fuel delivery---I.E.: fuel pumps, filters, delivery lines, pressure regulators, injectors, ad nauseum......

some of them are relying on reprogramming the later 32V computers to up the fuel delivery, but there is only SO much to be had with the stock 928 system.

similarly, Carl Fausset is the only one I have seen to address O-ringing the heads, to prevent head gasket blowout. nobody else is even talking about it, although I have raised the issue before. Mark has made some mention of changing his head gaskets to a multi layer unit, but has not recommended this on his turbo kit installation. Herr-Kuhn is still in the R&D stage, so it waits to be seen just how much he can crank out of his twin hairdryer unit before it gives the head gaskets the good old "heave--ho".... --- he's built some good stuff in the past, but it's been pretty tame compared to what he's working on now.

supercharging is old skool hotrod stuff. turbocharging is relatively newer, with intercooling coming about in the last couple of years.

My point is this----

if you are really serious about building a 928 motor that will melt a set of 12 inch wide tires and push the top speed through the 200+ mph range, using some sort of blower, you'd better O-ring the heads and build a high volume fuel delivery system to prevent starvation under high boost conditions.

If you don't, as many are not, you're just playing with a hand grenade that's got the pin out of it.


and yes, every one of these kits suppliers will probably disagree with me, but that's their privilege. produce kits for the masses, and hope the customer will have enough sense to not blow it up under stress.....it's a great thing, but sticking a blower on a 25 year old engine without building it (and the car) for the extra abuse is just nonsense. If I spent $6 G's of my hard earned money for a blower, I'd make damn sure I had a motor/car to support it.

truth is, a 300 horse shot of nitrous will give you the same results for $5 thou less coins.....see what I mean?

just my opinion, nothing more.

[Mule]

This is not complicated. If an S4 motor makes 320 with a correct fuel air ratio, 1 bar provides (basically) a 100% increase in air, if the fuel air ratio remains correct how can the hp increase more than 100%?

[Herr-Kuhn]

Mule, it is possible. You are filling the cylinder more before you even start to crank into boost. I built a twin turbo 4.5 liter 928 which made around 375 rwhp on 10 or so psig. Consider the turbocharger increases the VE of the engine before it ever starts to build pressure. If I pull off the turbocharger lines to the plenum on my car while it is running you can feel a steady flow of air out of the turbochargers...you are no longer just "sucking" the air in. For lack of a better explaination the turbo is acting as a small fan even at idle...when you get into boost it is really cramming it in hard. If you can keep the exhaust back pressure low enough you can make serios HP. If you can get the system into the crossover point where the exhaust back pressure is lower than the intake pressure you will make SERIOUS power. Most street setups will never operate here, but more likely in the 1.5X to 2.0X range.

As I stated in the other post, there are a ton of variables here, perhaps the wildest being the Dynojet and the inflated numbers it spits out compared to Mustang and other true loading dynos. Anybody who claims the dynojet is a real number doesn't understand all the parameters. We all use it because it is a baseline for measurement and the basis for competition. If I show up with a Mustang dyno sheet my power could be down 50-75 HP over what the dynojet may show. True story...the shop which ran my car on the Mustang dyno told me they had a 4 cyl Honda which made 430 Wheel HP on their dyno...the owner took the same car, added one psig of boost and put the car on a Dynojet and it was over 500 WHP. One psig on a 4 cylinder will never yield those results...the dynos just return different numbers. People use the dynojet because it gives the highest numbers, it has been proven time and time again.

Also remember, a lot of people are not just adding boost, but making other changes at the same time. Things like dropping cats, bigger exhausts, camshafts, etc.

As for the headgaskets, I have to respect your opinion, but I somewhat disagree. I have a 1993 Audi which runs 24 psig with 275,000 on the original engine (never opened). I have never had a head gasket problem. I agree O-ringing is a good thing for all out assult performance boost levels, but when you are talking 10-14 psig it is not necessary and is often a cure to the wrong problem (i.e. detonation). 8 turbocharged psig on an S4 engine becomes an all out weapon on the street. The turbocharger speaks very loudly at this level in terms of transient response and major torque to the wheels.

[Mule]

I think you hit on the real answer, the dynos. Remember, no engine has ever "sucked in " any air. It was pushed in under 14.7 lbs per sq in of "atmospheric" pressure. I don't believe anyone can show that 29.4 lbs or one bar can more than double the hp. If someone has some info to show it can I'd be interested to look at it.

[Danglerb]

If the before and after is on the same Dynojet, the absolute HP numbers may not be accurate, but the since the same error factor applies to both the change, at least the ratio should be reasonably accurate, like 5% or so.

Unless you are totally loaded with money, or totally lucky a NA engine isn't optimized for the input air pressure (as in now lets build the engine again and try .1 more compression). Its close with some safety margins, allowance for altitude and weather. Boosted engine on a dyno can be tuned right to the edge, and since you are working near detonation etc issues related to it can be explored and perhaps improved.

Why get too stuck on claims, look at the system, look at the testing and methods, refinance the house and move forward.

[Red Baron]

Does anyone know of a place/shop doing out of car dyno's on 928 motors?

Mule and I had a conversation about some of the dyno HP numbers on rennlist, and that is how this came up. With the loss for turning the blower, I have hard time believing the HP/TQ numbers people are posting.

It's interesting that simply using a different dyno can result in such a difference in HP.

[Herr-Kuhn]

In the end all that really matters is which vehicle outperforms the rest. "Outperforms" can mean a lot of things from an all out number to the way if feels when you drop the hammer to the fuel economy it delivers...or it could be a combo of any number of things which are important to you. I have yet to see anybody win a competition with a dyno sheet. This is why I have argued that powerband width is far more important than having a high peak HP number. If you make a decision based on peak HP you have not considered all the facts, you have to look at the total power curve and how it relates to how the vehicle performs on the road against gravity (hills, etc.) and aerodynamic loads. For example if two cars make the same peak HP number yet one delivers +50-100 HP in mid to high end areas of the power curve it is obvious which one will outperform in the real world. People do fixate too much on the "HP" number.

Real world performance is all that matters.

[Mule]

My guess is that 550 rwhp should net low 11 second et's with roughly 3600 pounds.

[Danglerb]

ET is a number mostly related to drag racing skill, and setup for drag racing. Drag slicks vs street radials can be good for a full half second on ET. I prefer MPH, which shows Hp to the ground very well with less abuse in the launch required for good numbers.

I don't like the idea of turning a 928 into a drag car, if I wanted that, for less then $5k my 91 Mustang would be my choice.

What I would really like to see are average lap times in a 20 minute or longer session. See how the systems perform with extended load and G forces. Plenty of boost systems barely make it through a 1/4 mile before thermal issues get ugly.

[Mule]

Top speed has it's drawbacks ie General Carl from renlist. He & his wife were lifeforced to intensive care when a part failed on a 200 mph blast in his 911.

[Herr-Kuhn]

The true HP formula is as follows:

Power=Torque*Rotational Speed

550 ft-lbf/sec=1HP

So if you have a car making 300 ft-lbs at 6000 RPM, the equation would work as follows:

(300 ft-lbf)*(6000 rev/min)*(2Pi Radians/rev)*(1min/60sec)=188,496 ft-lbf/second

188,496 ft-lbf/sec *(1HP/550 ft-lbf/sec)=343 HP.

Most people use the "5252" rule....HP*TQ/5252.

300*6000/5252=343 HP

The 5252 comes from the following:

(2Pi radians/rev)*(1min/60sec)(1HP/550 ft-lbf/sec)=1.90399...1.90399^-1 power ="5252"

It is like a little torque/speed/HP traingle. All three are inter-related.

[Herr-Kuhn]

Mule, I don't think you can generalize performance by a stated rwhp number.

Take two cars, each making 550 RWHP with one having 50-100 HP more lower in the rpm ranges (i.e 3500 rpm up to redline). The shape of the power curve tells a lot more about how the car will drive out on the road as compared to a peak rwhp number.

[Danglerb]

Frankly road race high speed scares the beans out of me, for good reason its very dangerous. If I ever do a track car I suspect it will be more Jim Bailey OB than something boosted and really fast.

Traction should keep most 928 based cars below the dangerous speeds in a 1/4 mile run, and thats the MPH I was refering to, not salt flat top speed, which IMHO is one small step of crazy away from playing chicken.

Truth is I am much less of a performance junky than a fun junky. I want a street car thats fun to "horse" around with, not a ballistic missile ticket magnet. I see a sweet spot in a street car around 0-60 in 5 seconds with a fairly soft non wheel spinning start, and optimal fun/trouble balance.

[Herr-Kuhn]

In this case DanglerB...you would not want my twin turbo.

[Danglerb]

Too many cars in SoCal take the fun out street racing for sane people. Car in the body shop I was in last week was a fully built Chevy 572 going in an old Chevelle, nothing I can think of doing to a 928 is going to touch that, and its going to be his daily driver.

Have fun, and fly low and avoid the radar, is my plan.

OTOH all this turbo talk is giving me the itch to fly something.

[Mule]

That 572 is 600 hp. He might have a well done turbo 928 by a little in the quarter. When the road turns???????

I know a guy that was driving an 8.80 mustang to work. I don't think anybody here is trying to do that. Low 11's, high 10's I think is a reachable goal, Remember, current Z06's go elevens just as they come out of the showroom, 505 hp, 3080 lbs.

[Danglerb]

I am pretty sure the guys with 450 rwhp spinning their tires aren't really having fun, it just seems like it to them since they don't know the real happiness that only comes from a balanced sports car.